As from raduino v1.20, it is no longer necessary to download and install the Si5351 library. The routines that drive the SI5351 are now included in the sketch.
Earlier raduino versions required the external Si5351 library written by Jason Mildrum, NT7S. The simplest way to make this library available to the Arduino IDE is to use the built in Library Manager function.
On the IDE menu, click the "Sketch" menu item, then select "Include Library", and finally, choose "Manage Libraries." From the Library Manager window which appears, type "si5351" in the search box at the top of the window.
Click on the result the is titled "Etherkit Si5351" and click the "Install" button. The Arduino IDE will reach out to the GitHub repository for the library, retrieve all of the required files, and store them in the correct location for use. As a side benefit, the IDE will keep an eye on the repository for changes to the library and offer to update your local copy. Be warned, updating libraries may sometimes cause the Raduino code to fail to compile!
If other external libraries are ever included in the Raduino code base, these steps can be used to locate and download the necessary library files. Simply search for the name of the library and click the install button.